Add adminAccount.username, decoupled from the seeded admin's hardcoded name

The seeded admin account's UserModel "name" was hardcoded to "admin".
Make it a separate, overridable option (still defaulting to "admin"
for compatibility) -- it's purely a display name, since HortusFox logs
in by email rather than this field.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -69,6 +69,7 @@ and a `services.hortusfox-web` NixOS module that wires the built package into a
 | `database.name` | string | `"hortusfox"` | MariaDB database name. |
 | `secretFile` | null or path | `null` | File of `KEY=VALUE` lines (`APP_KEY`, `SMTP_PASSWORD`, API keys, ...) appended verbatim onto the generated `.env` at runtime — not parsed, so it can carry arbitrary keys. Keep it out of the Nix store in production (agenix/sops-nix); a store path is fine for throwaway test configs. |
 | `adminAccount.passwordFile` | null or path | `null` | Plaintext password file for a seeded initial admin account. HortusFox has no CLI/seed hook of its own and its only "create user" flow lives behind the admin panel, so without this a fresh deployment has no way to log in at all. Only takes effect on first boot; changing it later has no effect on an existing account. |
+| `adminAccount.username` | string | `"admin"` | Name for the seeded admin account — purely a display name, since HortusFox logs in by email rather than this field. |
 | `adminAccount.email` | string | `"admin@${domain}"` | Email for the seeded admin account. The default is just a guess at a real mailbox — set it to an address you actually control if you intend to use HortusFox's "Restore password" flow, which emails it. |
 | `settings` | attrs of str/int/bool | `{ }` | Extra/overriding `.env` entries, merged over the module's generated defaults (`settings` always wins). Note `php asatru` (used for migrations) only runs when `APP_DEBUG` is true, so migrations silently no-op unless it's set here or via `secretFile`. |
 | `smtp.host` | string | `""` | SMTP server host. |